A DFT Study of the Al2O3 Atomic Layer Deposition on SAMs: Effect of SAM Termination

The atomistic mechanism of the initial atomic layer deposition (ALD) reaction of trimethylaluminum [Al (CH3)3, TMA] on octadecyltrichlorosilane (OTS)-based self-assembled monolayers (SAMs) with different functional termination groups was investigated using density functional theory (DFT). The adsorption of TMA on -NH2-terminated OTS involves formation of a dative bond between the amine lone pair and the Al atom of TMA and is exothermic by 0.86 eV. Conversion of the dative bond to a covalent bond through a four-centered H transfer from the NH2 to the methyl group of TMA to form methane involves a barrier that is 1.22 eV above the adsorbed complex and is exothermic by 0.94 eV. Reaction between TMA and the -OH-terminated SAM results in an adsorption energy of 0.60 eV. Although the dative bond is weaker, the barrier to converting the dative bond to a covalent bond to form methane is only 0.65 eV relative to the complex structure and the overall reaction is exothermic by 1.44 eV. The reaction of TMA with the methyl-terminated SAM forms no adsorbed complex and the ligand exchange reaction to form methane requires a barrier of 1.82 eV and is endothermic by 0.09 eV. These results indicate that the end group of the SAM significantly affects the selectivity toward TMA adsorption and subsequent ALD of Al2O3. Although initiation of ALD reactions involving TMA appears to require lone pairs, even strong dative bonds, such as those between amine groups and TMA, lead to ligand exchange reactions that are not kinetically favorable